ISDN is a relatively newly developed and emerging field of telecommunications which integrates computer and communications technologies to provide, worldwide, a common, all-digital network. This is based, in part, on standardizing the structure of digital protocols developed by the Internacional Telegraph and Telephone Consultative Committee (CCITT). Despite the implementation of multiple networks within national boundaries, from a user's point of view there is a single, uniformly accessible, worldwide network capable of handling a broad range of telephone, data and other conventional and enhanced services.
A complete description of the architecture of ISDN is beyond the scope of this specification. For details, and for an extensive bibliography of references on ISDN, see Stallings, ISDN, An Introduction, MacMillan Publishing Company, 1989.
An ISDN is structured by architecture closely following the OSI Seven Layer Reference Model. Within the framework of ISDN, the network provides services and the user accesses the services through the user-network interface. A "channel" represents a specified portion of the information carrying capacity of an interface. Channels are classified by two types, Basic Rate ISDN (BRI) and Primary Rate ISDN (PRI). BRI delivers two B-channels, each having a capacity of 64 Kbps, capable of transmitting voice and data simultaneously. A 16 Kbps D-channel transmits call control messages and user packet data. PRI provides twenty three B-channels of 64 Kbps capacity each for carrying voice, circuit switched data or packet data. The D-channel is a 64 Kbps signaling channel. The B and D channels are logically multiplexed together at Layer 1 of the OSI Reference Model.
FIG. 1 depicts the conventional ISDN interfaces. At the customer premises, an "intelligent" device, such as a digital PBX terminal controller or Local Area Network (LAN), can be connected to an ISDN terminal TE, such as a voice or data terminal, which is connected to a Network Termination (NT1). Non-ISDN terminals TE may be connected to a Network Termination (NT2) and a Terminal Adapter TA. The NT2 in turn is connected over an "S/T-Interface", which is a four-wire bus, to a termination NT1 that performs functions such as signal conversion and maintenance of the electrical characteristics of the loop.
At the local loop, a two-wire bus, termed the "U-Interface", or "Loop", interconnects NT1 and a Loop Termination (LT) at the central office. Finally, the "U-Interface" is a bus between the local loop at the carrier end and exchange switching equipment. Details of this architecture are provided in ISDN: An Overview, Data Pro Research, Concepts & Technologies, MT 20-365; pp 101-110, published by McGraw Hill, Incorporated (December 1988).
For connecting a personal computer to a conventional telephone line, a modem is required to convert outgoing digital signals generated by the computer into analog signals to be carried by the line, and to convert incoming analog signals to digital signals. Because an ISDN line is a digital network, however, no modem is required to interface a computer with the line; a computer is able to be connected to the ISDN line directly.
On the other hand, any interface between a computer and the ISDN must carry out conversion between the protocol stack implemented by the computer and ISDN protocol. The present invention provides gateway functions between personal computers and ISDN lines in a manner supporting existing communication protocols.
An interface may operate at any of several layers of the OSI model. A "repeater" is an interface operating at the physical link layer. A "bridge" interconnects networks at the data-link layer, and a "router" functions at the network layer. "Gateways" handle higher-level internetwork protocols. This terminology is not universal; for example, "gateway" sometimes is used to describe a "router", and it occasionally is used to refer to a "bridge". The term "gateway" will be used hereinafter to refer generically to any of these devices.
An ISDN gateway must satisfy several functions. It must be capable of transferring files on the ISDN at a very high rate of data transfer. The gateway furthermore must be "transparent" to the user, that is, the user of a computer should not be able to distinguish between data transfer among local resources and remote data transfer over the ISDN. Furthermore, standard communications software available for personal computers should be application to communications on the ISDN. Bandwidth utilization of the Network must be efficient and independent of the amount of traffic encountered.
Currently, gateway functions for personal computers are carried out by central office equipment in accordance with customer specifications. A need exists to establish gateway functions with equipment installed at customer premises to enable computers to be universally connectable to the ISDN. A further need exists to establish gateway functions having the characteristics described in the preceding paragraph.